1. Field of the Invention
The present invention relates to surface acoustic wave devices (hereinafter referred to as SAW devices) such as filters, resonators, and oscillators, which use surface acoustic waves traveling on a flat surface of a piezoelectric substrate and the manufacturing method therefor.
2. Description of the Related Art
The surface acoustic wave (also referred to as SAW hereinafter) has wave energy that concentrates only near the surface of a medium (an elastic body) for propagation. Therefore, the SAW device has an advantage in its suitability for generating, detecting, and controlling waves on the surface of the body. It also has an advantage of miniaturizing apparatuses that employ the SAW devices due. to the wave velocity (acoustic velocity) of a few km/s. Thus, in recent years, SAW devices have been widely used in electronic communication parts.
FIG. 1 shows an example of the SAW device that is used in electronic communication parts such as a high-frequency filter. Using the SAW device requires a piezoelectric substrate 1. A pair of input electrodes 2 formed in the shape of a comb, which are formed on the piezoelectric substrate 1 converts an electric signal, which carries information, into a SAW. In addition, a pair of comb-shaped output electrodes 3 converts the SAW into an electric signal. The electrode fingers 4 of the comb-shaped output electrodes is formed in an alternate arrangement on the piezoelectric substrate 1. The SAW propagates in the direction of the parallel electrode fingers 4. A piezoelectric substrate having a larger electromechanical coupling factor, which is indicative of the efficiency of electromechanical conversion, provides higher conversion efficiency. Thus, a piezoelectric crystalline material having a large electromechanical coupling factor is desirable. A piezoelectric crystalline material substrate such as LiNbO3 or LiTaO3 is used. This material is an oxide single crystal perovskite-based material comprised of lithium (Li), potassium (K), tantalum (Ta), niobium (Nb) and oxygen (O).
It has been recently discovered that a single crystal of KNbO3 (hereinafter referred to as KN) has an electromechanical coupling factor ten times larger than that of LiNbO3 or LiTaO3 crystalline material. However, it is difficult to produce in volume the bulk crystal of KN at low cost, and thus research has been conducted to make a thin film crystal of KN. Accordingly, it may be conceivable to obtain thin KN films by means of a conventional deposition method such as the liquid phase epitaxial method (LPE) or the sputtering method. However, by the LPE method, it is difficult to accurately deposit a thin film of a thickness in the order of microns. In addition, the sputtering method is good at controlling the thickness of the film, however, it has a problem in that it is difficult to combine targets to obtain a thin KN film having target compositions and the substrate is liable to be damaged during the deposition process.
With such prior art deposition methods, it is difficult to grow KN films and thus in most cases, SAW devices employing such crystals can be costly.
Accordingly, an object of the present invention is to provide a SAW device having thin piezoelectric crystalline films of ferroelectric crystal, which are formed on a crystal substrate by a thin film crystal forming means such as the metal organic chemical vapor deposition (hereinafter also referred to as MOCVD) method. Furthermore, another object of the present invention is to provide a device comprising the piezoelectric crystal film with the perovskite structure such as KN having an orientation suitable for the SAW device and the manufacturing method therefor.
A surface acoustic wave device of the present invention comprises a crystalline substrate having a structure selected from the group consisting of the perovskite structure, the spinel structure, and the rock salt structure. The device also comprises a thin piezoelectric crystalline film having a perovskite structure and deposited on said crystalline substrate by chemical vapor deposition, and an electrode means for generating a surface acoustic wave on said thin piezoelectric crystalline film. The device is characterized in that a surface of said crystalline substrate on which said thin piezoelectric crystalline film is deposited is a mirror surface inclined at an offset angle xcex8 from the (001) plane of said crystalline substrate, and said electrode means have electrode fingers arranged in parallel to each other so that a surface acoustic wave propagates along one direction of crystalline axes of said thin piezoelectric crystalline film.
In an aspect of the present invention, said thin piezoelectric crystalline film is composed of KNbO3.
In another aspect of the present invention, said crystalline substrate is composed of SrTiO3.
In a further aspect of the present invention, said offset angle xcex8 is set within a range of xe2x88x9210xc2x0 less than xcex8 less than 10xc2x0 (xcex8xe2x89xa00xc2x0).
In still another aspect of the present invention, said thin piezoelectric crystalline film crystal is grown so that a-axis is oriented in a surface of growth layer thereof.
In a further aspect of the present invention, said crystalline substrate is composed of MgAl2O4.
In another aspect of the present invention, said crystalline substrate is composed of MgO.
A method for manufacturing a surface acoustic wave device according to the present invention is characterized by comprising the step of forming a mirror surface inclined at an offset angle xcex8 from the (001) plane of a crystalline substrate with a structure selected from the group consisting of the perovskite structure, the spinel structure, and the rock salt structure. The method also comprises the step of growing, by chemical vapor deposition, a thin piezoelectric crystalline film with the perovskite structure on the inclined mirror surface of said crystalline substrate. Furthermore, the method comprises the step of forming electrode means having electrode fingers arranged on said thin piezoelectric crystalline film in parallel to each other so that a surface acoustic wave propagates along one direction of crystalline axes of said thin piezoelectric crystalline film.
According to the aforementioned surface acoustic wave device and the manufacturing method therefor of the present invention, surface acoustic wave devices comprising a thin piezoelectric crystalline film of the perovskite structure such as KNbO3 with a large electromechanical coupling factor can be manufactured at low cost.